Circuit arrangement for producing electrical pulses



1943- P. K. CHATTERJEA ET AL 2,

CIRCUIT ARRANGEMENT FOR PRODUCING ELECTRICAL PULSES Filed March 25, 1943 23 GAS l2 DISCHARGE l3 VALVE J 2 mTl. 2

l atentecl Dec. 28, 1948 UNITED (STATES .ZPATENT UF CIRCUIT GEMENT FOR PRODUCING ELECTRICAL PULSES Application March 23, 1943,Serial No. 480,156 InGreat' Britain March 27, 1942 4 Claims. -1

"The present invention relates to electrical ipulse transmission systems, and particularly to double'pul'se systems in "which the original pulses have to be re-con'stituted in the receiver of the system. It 'is of special application'to time modulated pulse systems.

In thesesys'tems, in orderto effect an economy 'in'power, the time modulated'pulses are frequently'not themselves transmitted, but'each is represented by two short'rmarking pulses transmitted respectively at thebeg'inning and end thereof. The original time modulated pulse generally 1 has to be re-constituted from the two marking pulses, in the receiver. Various methods of performing this operation havebeen suggested, generally requiring twoormore'valves, and 'are usu? ally of aperiodic 'or additive nature. By the method of the present invention a simpler arrangem'entbecomes possible using only one'valve, p'referably'of the gas-discharge type, or'a de- 'vice having similar characteristics; the arrange- --ment is aperiodic in'nature, and thusfis independent of the pulse repetition :frequency. The .in- "vention'is' however also applicable tos'omeof the knoWn-arr'agnements employing; for-example, cir- -:cuits'of themultivibrator type and the like.

Although a time modulated :pulse system has been assumed for the purpose'ofexplaining the invention, it is equally applicable in all cases vwhere the transmittedpulses are defined by pairs ofmarkingpulses. For" example the synchronizingpulses in aitel'evision system might be trans- -mitted this may, and could be'reformed .by the ---methocls 30f the invention.

According to the invention, there is provided' a receiving arrangement for re-constituting a pulse from tWO m'arking pulses in an electrical =d'ouble pulse transmission system, comprising a device or circuit having two electrically .stable "conditions, means for deriving two differential :pulses of'opposite 'sign'from each marking pulse, and means for applying the differential pulses to "switchthed'evice' or circuitfr'om one stable conadition tothe other.

The "invention will be more clearly understood from the following detailedldescri'ption and by reference to the accompanying drawings in which:

Figs. :1 and 4 show schematic circuit diagrams :of .twoiarran'gements according, to the invention; and

"Figs. 2, 3, 5 and 6 show wave forms of pulses used' 'for' explaining the operation 'of the circuits "of the invention.

- Itwis well known "that in gas-discharge devices.

there is a critical plate voltage at which the'gas becomes ionized, and a discharge occurs. "Beference can be increas'ed by providing the valve with a suitably placed --and'biassedcontrol electrode or grid.

If a sudden voltage change be'a'p'plied'to a condenser and-resistance connected in series,- and provided that the time constant of the circuit'is relatively small, a so-called 'diiferentiation -efiect occurs, whereby a voltage pulse is generated across the resistance whose instantaneous'amplitude'is'de'termined by the rate of change of the applied voltage. Such a pulse ma "be termed for convenience a difierential pulse.

The invention makes use of such differential pulses'preferably in combination with a gas discharge valveof the kind described, to reconstitute the pulses in a time modulation system, for

example, from the pairs of marking pulses which define them. The valve is also preferably provided a control gridfor increasing the difference between the critical voltages as explained above.

Fig; 1 shows in simplified form the basic circuit arrangement according to the invention. A gas-discharge valve 1 has its plate 4 connected through a resistance R to the positive terminal of the hightension supply,through a condenser C'to one input terminal 2, and directly to an output terminal 5. The cathode, and'the other input and output terminals 3 and 8 are connected to the negative terminal of the supply and prefer-ably also to earth. A large by-pass condenser l .is connected across the supply for .the purpose of connecting the resistance R effectively to earth for-transient currents, so that R may be regarded as connected in parallel with the valve I.

r The voltage applied to the plate 4 of the valve I should be just below the critical ionizing voltage,sothat the'plate circuit'impedance will be large compared with the resistance R. As regards transient voltages appliedtothe input'terminals 2 and.3, the circuit connected to the input terminals can accordingly be regarded"; as

vinput terminals 2 and 3.

3 substantially equivalent to a condenser C in series with a resistance R.

Fig. 2 shows diagrammatically the forms of two short unidirectional negative voltage pulses which may, for example, be the two marking pulses which define one of the time modulated pulses. Fig. 3 shows the corresponding variation of the voltage across the resistance R. It will be seen to consist of two pairs of similar differential pulses of opposite sign, the first of each pair being produced by the leading edge of the corresponding Fig. 2 pulse, and the second by the trailing edge. The actual form of these differential pulses is modified by the time constant CR of the circuit. The effect of this time constant is seen chiefly in the curved trailing edges of the differential pulses which represent the charge and discharge curves of the condenser C.

The Voltage variation of the differential pulses shown in Fig. 3 will be added to the steady voltage applied to the plate of the valve I from the high tension supply, and accordingly the first efiect of the first marking pulse will be a reduction in the total plate voltage, produced by the first differential pulse. When the second diiferential pulse arrives, the plate voltage will be increased, and the conditions can be so chosen that this increase is sufiicient to initiate a discharge in the valve I. A relatively large current flows, and the impedance of the valve falls to a much lower value, and at the same time the plate voltage also falls, and will remain at a low-er Value until the second marking pulse arrives.

The change in impedance of the valve will reduce the effective value of the resistance connected in series with the condenser C. However, the new value of this resistance is arranged to be such that when the second marking pulse arrives, two more differential pulses will be formed. The first of these differential pulses will be used to extinguish the discharge and to restore the peak voltage to its original value. Its amplitude must therefore be sufiicient to reduce momentarily the plate voltage below the lower critical voltage.

It is also to be noted that the second positive differential pulse must not raise the plate potential above the upper critical voltage, otherwise the valve will ionize again. Actually when the discharge is extinguished the voltage of the plate 4 does not immediately rise to the original value on account of the time constant of the circuit comprising the condenser C, and the combined resistance of R and of the circuit connected to the This time constant must accordingly be sufficiently large so that when the second differential pulse arrives, the plate voltage has not reached a value high enough to enable the pulse to start another discharge.

The requirements which have been outlined can be easily satisfied by suitable choice of the resistance R, condenser C and the type and operating conditions of the valve.

It will thus be seen that the effect of the two marking pulses is to change the plate voltage of the valve I (and therefore also the voltage across the output terminals 5 and 6) from a high value to a low value for the interval between the pulses and to restore it afterwards to the original high Value. In other words, a pulse similar to the original time modulated pulse is produced at the output terminals 5 and 6.

Details of a preferred form of the basic circuit are shown in Fig. 4. The gas-discharge valve 10 comprises a plate II, a cathode l3 and a control grid I2. The plate II is connected to the positive terminal of the high tension supply through two resistances l4 and I5, the junction of which is connected to the input terminal 22 through a condenser 24. The cathode I3 is biassed positively from a potentiometer comprising resistances l9 and 2| connected across the high tension supply, the usual by-pass condenser 20 being provided.

The grid I2 is connected to earth through a resistance I8 and the output terminal 23 is connected to the plate ll through a blocking condenser 25. A large condenser 8 is connected across the high tension supply for earthing the resistance M as regards transient currents. The input and output terminals 1-6 and l! are each connected to ground.

In this circuit, the condenser 24 and resistance l4 correspond respectively to C and R in Fig. 1. Resistance I5 is provided for limiting the discharge current, and could be omitted if desired. It is however useful for assisting in fulfilling the necessary operating conditions outlined above. The by-pass condenser 20 is also not essential and may in some cases be omitted with advantage, but in general it has a stabilizing effect on the circuit.

The value of resistance 18 is not critical, but it shouldbe large enough to limit the grid current, but not so large that it prevents the grid potential from returning to the initial value more quickly than the plate potential when the discharge is extinguished.

The circuit of Fig. 4 operates substantially in the same manner as described in connection with Fig. 1, "and differs only in the provision of the control grid with its associated arrangements, and in the addition of the optional resistance [5.

Fig. 5 shows in detail the shape of the reconstituted timemodulated pulse obtained at terminals l! and 23. In this wave-form 26 is the first differential pulse which has no efiect on the Valve. 21 is the second differential pulse which ionizes the valve, so that the voltage of the plate I I falls rapidly at 28 and reaches the lower level at 30 with possibly a small differential pulse asindicated at 29, which however must not be great enough to extinguish the discharge. When the second marking pulse arrives the first of the corresponding differential pulses extinguishes the discharge as indicated at 3|, and the plate voltage rises rapidly. It does not however immediately return to its original value on account of the time constant of the circuit comprising the condenser 24 and the resistance l4 (including also the effect of the impedance of the circuit connected to the input terminals l6 and 22) as already mentioned. The voltage change follows the curve 32 until the second differential pulse reduces the voltage again suddenly at 33. The condenser then finally discharges as shown by the curve at 34 until the plate voltagev reaches the final value at 35.

The details of the leading and trailing edges of the re-constituted pulse depend on the values of the circuit-elements and also on'the duration of the marking pulses. These details are shown on a larger time scale in Fig. 6. At A in Fig. 6 is shown the leading edge with the various portions designated as in Fig. 5. If the resistance [5 in Fig. 4 should be too large, the lower corner may appear as the dotted line 35. At B, Fig. 6 is shown the preferable form of the trailing edge. The time constant of the condenser circuit should be such that when the second differential pulse arrives at 33, it brings the plate voltage just to the final value so that the upper corner will be substantially square, as shown by the full line. If the time constant is too small (Fig. 60), the voltage of the plate will have risen too high on the arrival of the differential pulse, which will carry the voltage beyond its final value, and may even initiate another discharge. If a new discharge does not occur, the voltage will return to the final value on the curve 36. A similar effect will be produced if the marking pulse is too long as shown by the dotted lines 31 in Fig. 6B. If the time constant of the circuit is too large, the plate voltage will not have risen high enough on the arrival of the second differential pulse, and the condenser will afterwards continue its discharge along the curve 34 (Fig. 6D).

It will be evident that the various details which have just been described will tend to have a distorting effect on the re-constituted pulse. Actually, of course, the duration of the marking pulses will always be chosen to be small compared with that of the original time modulated pulse, and so the actual distortion produced will be negligible. Accordingly although it is preferable to aim to produce a form such as is shown in Fig. 6B, small departures as shown in Figs. 60 or D will be permissible without much undesirable effect, so that the requirements are not very critical. Likewise, the resistance l5 (Fig. 4) is preferably chosen so that a substantially square corner is obtained at 35 in Fig. 6A, though a small amount of rounding will be of no consequence.

It will be appreciated that, according to the invention, each marking pulse is converted into the two differential pulses which are then used to switch a gas discharge valve between two stable electrical conditions.

It will be evident that the marking pulses may be amplified or otherwise treated in any suitable way before application to the arrangements described.

Various other modifications of the circuits according to the principles explained will occur to those skilled in the art.

What is claimed is:

1. A receiving arrangement for reconstituting a time modulated pulse from two pulses marking its leading and trailing edges respectively, comprising a differentiating circuit, means for applying said two marking pulses tosaid differentiating circuit, means for deriving from said differentiating circuit two differential pulses of opposite sign for each marking pulse, a gas discharge valve including a plate and a cathode, a steady high tension potential source connected between said plate and said cathode to make said plate positive, means for applying said differential pulses to the input of said valve to initiate a discharge in response to the first marking pulse and subsequently to extinguish such discharge in the valve in response to the second marking pulse the time intervals between successive initiation and extinguishment of the said discharge being fixed by the interval between corresponding successive pairs of differential pulses, and an output circuit for said valve.

2. An arrangement according to claim 1, wherein said differentiating circuit comprises a condenser connected in series with a resistance, means for applying said positive potential to said plate over said resistance and means for applying said marking pulses between said cathode and said plate in series with said condenser.

3. An arrangement according to claim 1 wherein said gas discharge valve is provided with a control grid connected through a leak resistance to the negative terminal of the high tension supply.

4. An arrangement according to claim 1 further comprising a limiting resistance connected between said differentiating circuit and said plate.

PRAFULLA KUMAR CHATTERJEA. LESLIE WILFRED HOUGH'ION.

REFERENCES CITED The following references are of record in the 

